Assessing C and H isotopic fractionation of aliphatic hydrocarbons during microbial degradation: microcosm studies and environmental implications

Abstract

Microbial degradation of hydrocarbons has been widely studied for remediation purposes. However, there are still unquantifiable factors that are not properly understood because of the limitation with the conventional techniques used. Stable isotope analysis has been applied as an alternative approach to the conventional method. The unique stable isotope fingerprints of hydrocarbon compounds make them viable to be applied in source- apportionment of an oil spill due to its stability undergoing various processes. On the contrary, changes in the stable isotope compositions of the hydrocarbon discovered to occur during physical and biological weathering makes them useful in the application of this method in monitoring the occurrence of in situ biodegradation and estimating the rates and extent of degradation. Hence, we carried out microcosm studies to monitor and evaluate the microbial degradation of the aliphatic hydrocarbons following an oil spill using compound- specific isotope analysis. The effects of microbial degradation on the isotopic compositions of the hydrocarbons were tested using Rayleigh model in order to assess the state and behaviour of the system in operation. Based on the data in this study, the findings indicated that carbon isotopic fractionation patterns were different from that of hydrogen isotopes. Small carbon isotope enrichment in the substrates of n-alkanes could only be detected in nC12 and nC13 after slight to moderate biodegradation. On the other hand, 2H enrichment in nC12 to nC19 occurred very early during the biodegradation process with high values of enrichment. The trends observed in the 82H shifts in the n-alkanes caused by microbial degradation, generally followed the Rayleigh model. In summary, CSIA had been successful in providing fundamental data on the effects of biodegradation by efficiently demonstrating the magnitude and direction of isotopic shift occurring in aliphatic hydrocarbons.